PHPStan Abstract Parent Generics for Dummies

2021-01-04

"If you can't explain it to a six-year-old,
you don't understand it yourself."

The documentation barely scratches this topic and examples are not very clear. Neither were responses on GitHub issues.

What is the use case? We have two classes - an abstract repository:

abstract class AbstractRepository
{
    public function get(int $id): object
    {
        // ...
    }
}

and children classes that extend it:

final class ProductRepository extends AbstractRepository
{
}

In the controller, we use ProductRepository to display product detail:

final class ProductDetailController
{
    public function __construct(
        private ProductRepository $productRepository
    ) {
    }

    public function __invoke(int $id)
    {
        $product = $this->productRepository->get($id);
    }
}

Pretty straightforward, right?

Now we'll try to solve the hard question...

What Type is $product?

$product = $this->productRepository->get($id);

The class method AbstractRepository->get() returns and object, so for PHP, PHPStan, Rector and IDE it's an object. Nothing less, nothing more.

But what do you see? You probably assume it's a specific object with a specific type - Product. How can we get this knowledge to the code?

1. We could add a method to ProductRepository

 final class ProductRepository extends AbstractRepository
 {
+    public function get(int $id): Product
+    {
+        return parent::get($id);
+    }
 }

2. We could add magic @method docblock

+/**
+ * @method Product get(int $id)
+ */
 final class ProductRepository extends AbstractRepository
 {
 }

3. We could add @var annotation to every $product variable

+/** @var Product $product */
 $product = $this->productRepository->get($id);

4. We could add assert() to $product variables

 $product = $this->productRepository->get($id);
+assert($product instanceof Product);

5. We could add instanceof check

 $product = $this->productRepository->get($id);
+if (! $product instanceof Product) {
+    throw new ShouldNotHappenException('$product is not a ' . Product:class);
+}

We can see these solutions out in the wild. All of them are valid. They verify an assumption - is object a Product type?

Validation !== Type Declaration

But validation is not designed for type specification. It's a layer to verify external input from QA when they walk in a bar order beer, -5, $^@'ł, an array of T_INF, and a bottomless glass of invisible water.

Here we know the $product is always the Product type unless some very nasty bug will get into Doctrine. How can we teach our PHP code this knowledge?

Generics!

Adding generics to 2 classes is 2-steps

1. Open Parent

First, we need to tell the abstract class to be opened to type override from children. By convention, the generics are not defined with the @generics keyword but by @template. It has nothing to do with rendering templates.

With @template we define a keyword and a type. Same as @param int $age does.

+/**
+ * @template TEntity as object
+ */
 abstract class AbstractRepository
 {
 }

Now we use this TEntity keyword in places where we know the specific type will be used:

 abstract class AbstractRepository
 {
+    /**
+     * @return TEntity
+     */
     public function get(int $id): object
     {
         // ...
     }
 }

Good job, we're halfway through.

2. Specify Child

So how do we tell ProductRepository to treat every object as Product?

+/**
+ * @template TEntity as Product
+ */
 final class ProductRepository extends AbstractRepository
 {
 }

That should be it, right? Let's run our controller and try it:

$product = $this->productRepository->get($id);
// "object"

Damn, what's going on?

I stuck on this part and could not go on. We defined TEntity in both of our classes. We defined TEntity as Product. What is wrong?

How to promote the TEntity type to Parent Class?

The problem is, PHPStan sees generic types only in the classes they're defined by default. While @return int gets promoted to child methods, @template does not.

• So TEntity is Product only in class we defined it in - ProductRepository
• And TEntity is only an object in class we defined it in - AbstractRepository

So we need to tell AbstractRepository to use TEntity as Product, without modifying it.

In the documentation, there is mentioned @extends annotation. It is not related to extending a class, but promoting types to parent class.

Let's get back to our repository:

 /**
  * @template TEntity as Product
  */
 final class ProductRepository extends AbstractRepository
 {
 }

The @extends annotation takes an argument of 1-n item names. These items will be pushed parent repository.

How do we tell the parent class to use TEntity as Product?

 /**
  * @template TEntity as Product
+ * @extends AbstractRepository<TEntity>
  */
 final class ProductRepository extends AbstractRepository
 {
 }

As we don't use TEntity in our child class, we can merge it to @extends (thanks Ondra for pointing that out in the comments):

 /**
- * @template TEntity as Product
+ * @extends AbstractRepository<Product>
  */
 final class ProductRepository extends AbstractRepository
 {
 }

Now the TEntity in AbstractRepository will be overridden by type defined here. So TEntity will be treated as Product ✅

Beware! Even though it looks like any array<shape>, it has nothing to do with arrays.

How to Propagate 2 and more Types?

Let's say you have 2 generics types in your abstract class:

/**
 * @template TEntity as object
 * @template TQuery as object
 */
abstract class AbstractRepository
{
}

The @extends tag can take multiple arguments, separated by comma - in the same order the types are defined in abstract class:

/**
 * @extends AbstractRepository<Product, ProductQuery>
 */
final class ProductRepository extends AbstractRepository
{
}

That's how you use generics with parent abstract class with PHPStan.

Happy coding!

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